This invention relates to a battery and, more particularly, to a battery structure that realizes a light, compact and thin battery having a high discharging capacity at a high current density and satisfactory cycle characteristics.
Batteries have been used long as a main power source or a backup power source for a variety of equipment. The demand for batteries has recently been increasing with the development of portable electronic equipment, such as cellular phones and portable personal computers. Primary batteries and secondary batteries are available according to use. As to secondary batteries having great convenience, high performance batteries such as lithium ion secondary batteries and nickel-hydrogen batteries have been attracting attention. The present invention will hereinafter be explained by referring to lithium ion secondary batteries the demand of which has been steeply increasing for use in portable electronic equipment.
Conventional lithium ion secondary batteries comprise a battery body that is a cylindrical roll of an electrode body or a stack of rectangular electrode bodies, the electrode body being composed of a positive electrode, a negative electrode, and a separator that is interposed between the two electrodes to serve for insulation and retention of an electrolyte. The battery body is put in a metal-made case so that the positive electrode, the negative electrode and the separator can be brought into intimate contact by the pressure of the case thereby to maintain the contact between each electrode and the separator.
An electrical contact can be maintained by putting the battery body in a metal-made case, but there is a problem that the case, being made of metal, increases the weight of the battery. Moreover, it is difficult to make a thin metal case. Difficulty in making a thin case has been a great obstacle to fulfillment of the demand for batteries to be used in compact portable equipment.
In this connection, U.S. Pat. No. 5,437,692 discloses a structure in which a lithium ion-conducting polymer is used as an ion conducting layer, and a positive electrode and a negative electrode are joined to the ion-conducting layer with an adhesive layer containing a lithium compound. The inventors of the present invention previously proposed in Japanese Patent Application No. 8-338240 a battery structure requiring no metal-made rigid case and a process for producing the same, in which a positive electrode and a negative electrode are previously joined to a separator with an adhesive resin.
The method comprising joining a positive and a negative electrode to a lithium ion-conducting polymer with an adhesive layer containing a lithium compound is disadvantageous in that the adhesive layer attains weak adhesion and becomes considerable resistance to ion conduction. The method comprising previously joining a positive and a negative electrode to a separator with an adhesive resin has made it feasible to maintain an electrical contact among them without imposing an external force but involves a problem that resistance between electrodes (i.e., a positive electrode and a negative electrode) increases because of the existence of not only the separator but the adhesive resin between the electrodes.
Direct bonding of electrodes is a conceivable method. In this case, it is necessary to secure both ion conduction and electron insulation between electrodes that ought to be undertaken by a conventional ion-conducting polymer or separator, and yet to join the electrodes firmly.
Further, the conventional ion-conducting polymer has weak adhesive strength, and the conventional adhesive resin joining electrodes and a separator has high adhesive strength but tends to deteriorate ion conductivity.
Where the conventional adhesive resin is used, it turned out that cases are met with in which a solution of the adhesive resin is absorbed by a porous electrode only to exhibit low adhesive strength or even fail to bond.
In addition, electrodes have their surfaces smoothed by pressing but still have unevenness of several microns to form vacancies where a separator and the electrodes are not in contact. The vacancies that should have been filled with an electrolyte may get starved of the electrolyte, which depends on the amount of the electrolyte supplied and the condition of use of the battery. Starvation of the electrolyte leads to an increase of internal resistivity of the battery and reductions in battery characteristics.
The present invention has been reached, aiming at settlement of the above-described problems. It is an object of the invention to provide a light, compact and thin battery in which a positive and a negative electrode are joined firmly to maintain the adhesive strength while securing both electron insulation and ion conduction between electrodes and decreasing resistance between electrodes, i.e., internal resistance of a battery, to improve battery characteristics.
A first battery according to the invention comprises a battery body having a positive and a negative electrode containing an active material, an electrolytic solution containing an electrolyte, and an adhesive resin layer which is interposed in between the positive electrode and the negative electrode and is joined to at least one of the positive and the negative electrodes, wherein the adhesive resin layer comprises at least one layer and contains fillers. In this structure, at least one of the positive electrode and the negative electrode is directly bonded to the adhesive resin layer, and the filler added makes the adhesive resin layer porous. The electrolyte and an adhesive resin solution can be held in the pores. As a result, internal resistivity of the battery can be reduced while maintaining adhesive strength to provide satisfactory battery characteristics.
A second battery according to the invention is the above-described first battery, wherein the electrolyte is an organic electrolyte containing lithium ions. This mode, when applied to lithium ion secondary batteries which are required to have reduced weight and thickness, provides a high performance compact battery.
A third battery according to the invention is the above-described first battery, wherein the average particle size of the filler is equal to or smaller than the particle size of the active material of the positive and negative electrodes. According to this mode, when the positive electrode and the negative electrode are joined with an adhesive resin, an adhesive resin solution having the adhesive resin dissolved in a solvent is hardly absorbed by the electrode active material so that it can be held as an adhesive resin layer to give necessary adhesive strength.
A fourth battery according to the invention is the above-described first battery, wherein the average particle size of the filler is 1 xcexcm or smaller. According to this embodiment, the filler manifests a proper thickening effect for the adhesive resin solution to make the adhesive resin solution hardly absorbable by the active material thereby securing necessary adhesive strength. Further, the filler makes the adhesive resin layer porous to improve ion conductivity thereby providing satisfactory battery characteristics.
A fifth battery according to the invention is the above-described first battery, wherein the sum of a volume ratio of the adhesive resin and that of the filler per unit volume of the adhesive resin layer is less than 1. This mode secures the porosity of the formed adhesive resin layer thereby securing satisfactory ion conduction.
A sixth battery according to the invention is the above-described first battery, wherein the sum of a volume ratio of the adhesive resin and that of the filler per unit volume of the adhesive resin layer is 0.2 to 0.8. According to this embodiment, the voids of the porous adhesive resin are filled with the electrolyte to exhibit sufficient ion conductivity.
A seventh battery according to the invention is the above-described first battery, wherein the filler comprises at least one of non-conductive materials and semiconductors. According to this mode, the adhesive resin layer can be made porous to provide excellent battery characteristics including satisfactory ion conductivity while retaining adhesive strength.
An eighth battery according to the invention is the above-described first battery, wherein the adhesive resin layer comprises a layer containing an electrically conductive filler and a layer containing at least one of non-conductive fillers and semiconductive fillers. According to this embodiment, internal resistivity of the battery can further be diminished by the conductive filler-containing layer.
A ninth battery according to the invention is the above-described first battery, wherein the adhesive resin layer is constituted so as to fill the unevenness on the positive and the negative electrodes. This structure is effective in increasing the adhesive strength and preventing reduction of battery characteristics due to starvation of the electrolyte.
A tenth battery according to the invention is the above-described first battery, wherein the battery body is a laminate of a plurality of electrode bodies each composed of a single layer of the positive electrode, a single layer of the adhesive resin layer, and a single layer of the negative electrode.
An eleventh battery according to the invention is the above-described tenth battery, wherein the laminate is composed of the positive electrodes and the negative electrodes which are alternately interposed among a plurality of the adhesive resin layers.
A twelfth battery according to the invention is the above-described tenth battery, wherein the laminate is composed of the positive electrode and the negative electrode which are alternately interposed between adhesive resin layers and rolled up.
A thirteenth battery according to the invention is the above-described tenth battery, wherein the laminate is composed of the positive electrode and the negative electrode which are alternately interposed between adhesive resin layers and folded.